Applying Catalytic Recuperative Oxidation System to Process Airflow in Pure Terephthalic Acid Plant

During the formulation of Pure Terephthalic Acid (PTA), halogenated volatile organic compounds (VOCs) can be produced. Within the manufacturing process, methyl bromide is converted into hydrogen bromide. To this end, a leading chemical manufacturer was looking for a suitable pollution control system that can destroy VOCs. The pollution control system had to be designed in such a way that it can resist the corrosiveness of the hydrogen bromide and different organic loadings which, change the system’s temperature. Figure 1 shows the PTA pant.

Figure 1. PTA plant

Catalytic Recuperative Oxidation System

The manufacturer evaluated a number of suppliers and finally opted for Anguil Environmental Systems to solve its VOC emission issue. Due to the presence of methyl bromide and the high loadings of carbon-monoxide, a Catalytic Recuperative Oxidation System was selected, using a specialized catalyst designed by the Johnson Matthey Stationary Emissions Control Division. This catalyst was designed for PTA plant exhausts. Anguil's experience in integrating control devices into existing processes and successful catalytic oxidation of halogenated compounds were critical factors in the plant's decision.

Anguil’s catalytic oxidation system was specifically designed for 90,000 SCFM. Due to the presence of methyl bromide in the emission stream, additional design factors were taken into account. Methyl bromide was oxidized to hydrogen bromide with the help of the catalyst. In case hydrogen bromide drops below its dew point, it turns corrosive to the equipment. Equipment, downstream of the catalyst, must be specially designed to prevent "cool spots" where the hydrogen bromide can condense.

Steam Preheater

One possible "cool spot" is the heat exchanger. Without an appropriate design, the heat exchanger can reduce the air temperature coming out from the catalytic oxidizer to below the dew point of hydrogen bromide. To prevent potential condensation of hydrogen bromide and successive corrosion to the system, a steam preheater was integrated onto the incoming process stream prior to the catalytic oxidizer. The exhaust from the PTA plant is then heated by means of a 316L stainless steel plate and frame steam preheater. The preheated process air enters the tube side of the shell and tube heat exchanger. The elevated temperature of the process air entering the tube side of the heat exchanger inhibits the condensation of hydrogen bromide on the shell side of the heat exchanger, a crucial issue in the overall design of the system.

Since the process exhaust from the PTA plant differs in organic loading, the catalytic oxidizer design must accommodate these varying levels with nominal use of auxiliary fuel. This is achieved by using a bypass on the 316L stainless steel shell and tube heat exchanger. The heat exchanger bypass is closed under low organic loading conditions so that the full effectiveness of the heat exchanger is available to preheat the incoming stream.

Heat Exchanger Bypass

At high organic loadings, the catalyst’s outlet temperature is increased, resulting in a high preheat temperature. This increased preheat temperature may overheat and shutdown the system. Anguil’s "hot side" heat exchanger bypass controls the preheat temperature from the heat exchanger and prevents any high-temperature conditions.

Catalyst Technology

Choosing the right catalyst that would oxidize methyl bromide as well as carbon monoxide at low temperatures was crucial in this project. Traditionally, halogenated compounds, which comprise fluorine, chlorine, iodine and bromine, tend to have damaging effects on both base metal and noble metal oxidation catalysts. However, in recent years a number of advancements have been made in catalyst technology, resulting in the development of catalysts appropriate for the airstream under consideration. In this design, a platinum/palladium-based catalyst deposited on a ceramic substrate was used.

Anguil has designed a proprietary catalyst rack with specialty gasketing, which prevents the risk of gas bypassing the catalyst which would otherwise result in partial destruction.

Conclusion

Anguil’s Catalytic Recuperative Oxidation System is presently operating and meeting the regulatory compliance stipulated by the PTA plant. Following the success of this system, the chemical manufacturer procured a similar 90,000 SCFM unit. Additional systems were also installed for PTA manufacturers in the Southeastern United States and Middle East.

About Anguil Environmental Systems

Anguil Environmental Systems is a global provider of industrial air pollution control and energy recovery systems. The thermal and catalytic oxidizers supplied by Anguil are used to destroy Volatile Organic Compounds (VOCs) and Hazardous Air Pollutants (HAPs) that are by-products of various manufacturing processes. When released into the atmosphere these carcinogens are known to cause respiratory ailments, heart conditions, birth defects, nervous system damage and cancer in humans and animals.

In addition to their harmful effects on plants and trees, when left untreated VOCs and HAPs degrade in the presence of sunlight and contribute to low-lying ozone or smog. Anguil differentiates itself from the competition by offering all of the different vapor combustion technologies used for the destruction of these pollutants. This ensures an unbiased equipment selection for each application based on the destruction requirements, efficiency needs and process parameters. We not only design, manufacturer, service and install Regenerative Thermal Oxidizers (RTOs) but also direct-fired, catalytic and thermal recuperative systems. In addition, Anguil offers a wide variety of heat recovery and energy conservation technologies. Our energy recovery systems help manufacturers achieve greater energy efficiency, lower operating costs and reduce greenhouse gas emissions through the utilization of waste heat. The recovered energy is often used in process and comfort heating applications or converted into electricity.

This information has been sourced, reviewed and adapted from materials provided by Anguil Environmental Systems.

For more information on this source, please visit Anguil Environmental Systems.

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